Fatigue behavior of reinforced concrete beams strengthened with prestressed near-surface-mounted carbon-fiber-reinforced polymer strips

Fourteen reinforced concrete (RC) beams strengthened with prestressed near-surface-mounted (NSM) carbonfiber-reinforced polymer (CFRP) strips were tested under static and fatigue loads. The influence of various factors, such as the bond length, fatigue load amplitude, CFRP prestressing force, and plate end anchorage, on the load-carrying capacity of the strengthened beams was evaluated. Furthermore, the different failure modes observed, which included reinforcement fracture, concrete cover separation, and CFRP-concrete interfacial debonding, were investigated. Moreover, the applicability of the tensile reinforcement allowable fatigue stress in various design codes for NSM reinforcement systems was assessed. In addition, to predict concrete cover separation failure, an empirical model for the permissible plate end bending shear force was developed. The test results revealed that the CFRP-concrete interface bond strength was sufficient throughout the fatigue life of all the specimens, while concrete cover separation and reinforcement fracture were the primary failure modes of the strengthened beams under fatigue loads. The plate end bending moment and shear force increased due to an insufficient bond length (3100 mm) and a large load amplitude (64 kN), causing a rapid development of horizontal cracks and an eventual concrete cover separation. The critical factor causing reinforcement fracture was the reinforcement stress range in the stable growth stage (Sr2), rather than the initial reinforcement stress range (Sr1), which has not been distinguished in the existing literature and codes. Sr2 decreased as the fatigue load range reduced, or the prestressing force increased, and it was not related to the bond length or plate end anchorage. Thus, existing design codes overestimate the allowable reinforcement fatigue stress range, which is inapplicable to the structural design of RC beams strengthened with prestressed NSM CFRP strips.